Method for producing semiconductor films

ABSTRACT

A method for producing semiconducting films on a substrate wherein a plasma of a reactive gas is generated and an ion beam from the plasma is directed and accelerated toward a target of material of which the film is to be formed. The target which is maintained in a vacuum chamber at reduced pressure, is sputtered with the reactive ion beam and the material sputtered off the target is collected as a film on a substrate which is physically isolated from the plasma generating process and the sputtering process. Preferably the reactive gas is present in a mixture with an inert gas heavier than the reactive gas.

REFERENCE TO COPENDING APPLICATION

Reference is hereby made to copending application Ser. No. 254,341entitled Apparatus and Method for Producing Semiconducting Films filedconcurrently herewith in the name of Gerald P. Ceasar and Scott F.Grimshaw.

BACKGROUND OF THE INVENTION

This invention relates to methods of making semiconducting andphotoelectronic devices and in general to a method of coating asubstrate with a semiconducting material. More specifically, theinvention relates to the preparation of amorphous semiconductor filmssuch as amorphous silicon.

Semiconductor films are of some significance today for use in a varietyof industrial applications and are of particular interest for large areaelectronic applications such as solar cells, xerographic photoreceptors,thin film transistors, vidicons, etc. This interest is particularly highwith respect to hydrogenated amorphous silicon films for severalreasons. They have excellent photoelectric properties, high absorptioncoefficients through the visible region and are relatively low in cost.In addition, they can be manufactured in relatively large areas, are nontoxic, relatively hard and do not crystallize. Furthermore, they areambipolar and can be xerographically charged either positively ornegatively. They can also be doped to p or n type semiconductors totransport either positive or negative charge. In addition, they can bealloyed with other elements to provide material with tunable opticalproperties and which in particular are sensitive in the infrared region.With these properties they hold particular promise in the xerographicphotoreceptor area as a potential replacement for selenium and seleniumalloys. This is particularly true since they have the above superiorproperties and in addition are no more brittle than selenium.

This activity has been kindled by the discovery that the dangling bonddefects intrinsic in the preparation of amorphous silicon can be reducedand to some extent controlled by choice of film deposition conditions.In pure amorphous silicon these dangling bonds are present in a densityhigh enough to make films of this material unsuitable for manysemiconductor and photoelectric applications. The material, for example,cannot be successively doped and made into p-n operation devices. Theseintrinsic dangling bonds furthermore serve as recombination sites makingsuch films photoelectronically useless. In addition, the presence ofimpurities which introduce states in the band gap also alters theelectrical properties of the film. This is particularly noticeable withmetallic impurities in the film which give rise to electrically activeimpurity states in the band gap. If these states are present in highdensity it becomes impossible to control the conductivity of thesematerials thereby making n or p type doping not feasible. It has beenfound that if the amorphous films are formed in certain ways with thepresence of a reactive gas that the gas coordinates with the intrinsicdangling bond defects and thereby removes these localized states fromthe band gap. This a particularly true for silicon which has been foundto be especially reactive with hydrogen gas. These two techniques thathave been used in the prior art for this general procedure are the glowdischarge decomposition of silane and r.f. (radio frequency) or directcurrent sputtering with a reactive gas.

In the glow discharge chemical vapor deposition technique silane gas (SiH₄) is flowed between two electrodes, one of which has the substratemounted on it. As power is applied to the substrate the silane isdecomposed into reactive silicon hydrogen species which deposit as asolid film on both electrodes. The presence of hydrogen is importantsince it may coordinate with the dangling bonds in the silicon in partas the mono, di and trihydrides, and thereby serves to passivate thedangling bonds.

In the r.f. or d.c. (direct current) sputtering technique the substrateis fastened to one of two electrodes and a target of silicon is placedon the other electrode. Both electrodes are connected to a high voltagepower supply. A gas which may for example be a mixture of argon andhydrogen is introduced between the electrodes to provide a medium inwhich a glow discharge can be initiated and maintained. The glowdischarge provides ions which strike the target and remove by momentumtransfer mainly neutral target atoms which condense as a thin film onthe substrate electrode. The glow discharge also serves to activate thehydrogen causing it to react with the silicon and be incorporated in thedeposited silicon film. The hydrogen coordinates with the dangling bondsin the silicon to form mono, di and trihydrides.

Difficulties are encountered with both of these prior art techniquesprincipally because control over the several process parameters andthereby control of the plasma energy involved cannot be achieved ineither of the techniques. Particularly difficult is the reproduceabilityof accurate control of the discharge plasma since the floating potentialof the discharge plasma cannot be readily accurately measured orcontrolled. Further glow discharge processes are complex and dependcritically on a large number of process parameters. A glow dischargeconsists of a multiplicity of reactants, ions, free radicals, electronsand metastable excited species. These reactants may interact via amultiplicity of reaction pathways which may be initiated or propogatedin either the gas phase or glowing thin film surface. Furthermore, thesurface of the reactor walls and the floating potential of the plasmaexerts a well known but little understood influence on the plasmachemistry. These factors are moreover strongly affected by a number ofcritical deposition parameters including the r.f. power and frequency,flow rate, substrate temperature, pressure, concentration ratio ofgases, gas flow pattern and reactor geometry. The large number ofcritical process parameters and their complex interrelationships makesthese processes difficult to understand and control.

In both glow discharge or chemical vapor deposition and r-f-sputteringthe film is formed in the presence of either the plasma or thesputtering process thereby increasing the probability of the plasma orthe sputtering introducing defects in the film being formed since boththe plasma and sputtering process are inherently high energy destructiveprocesses. These process induced defects would arise from thebombardment of the thin film by the energetic species and radiationproduced in the glow discharge including excited and ionized moleculesand fragments, secondary electrons and photons. Furthermore, in both theglow discharge and r-f sputtering process the dangling bond sites ofsilicon, for example, may be coordinated with hydrogen as the mono, diand trihydride which is not desirable since the presence of the di andtrihydride in the film leads to undesirable photoelectric properties. Inparticular the photoconductivity is very poor. On the other hand it isdesirable to have the hydrogen exist as the monohydride since itpossesses excellent photoelectric properties. Furthermore, in the glowdischarge processes, the possibility of microvoids being present in thefilm also exists which further leads to degradation of the electronicproperties.

PRIOR ART

U.S. Pat. No. 4,213,844 to Morimoto et al is directed to an ion platingtechnique wherein the plating material is vaporized and wherein thesubstrate holder is an electrode. While it describes a "cluster ionbeam" there is no reactive ion beam generated which sputters a target ofthe material from which the film is to be formed. Furthermore, there isno sputtering at all since it relies on a vaporization process.

U.S. Pat. No. 4,217,374 to Ovshinsky describes a method of making anamorphous semiconductor film by vaporizing silicon, condensing it on asubstrate and preferably at the same time introducing two or threecompensating or altering agents like activated hydrogen and fluorine inamounts which reduce the localized states in the energy gap.

Ishii et al "Electrical Properties of Oxygenated Amorphous Si Preparedby Ion-Beam Sputtering;" Japanese J. Applied Physics, Vol. 18, (1979),No. 7, pages 1395, 1396 describes a duoplasmatron type ion source foruse in a sputtering operation where a target was sputtered with argonand O₂ was separately added to the chamber housing the substrate. Thereis no disclosure of sputtering with a reactive gas or more specificallywith reactive hydrogen. In addition the duoplasmatron inherentlyproduces a small beam focused to a very narrow degree and operates atrelatively high energy levels. In view of this, the duoplasmatrontechnique is capable of use only in small area applications, less thanabout one square centimeter, and is not suitable for use in large areaelectronic applications.

SUMMARY OF THE INVENTION

In accordance with the present invention an improved method forproducing semiconductor films on a substrate is provided. In particular,the method comprises generating a plasma of reactive gas and extractingand accelerating an ion beam from the plasma containing the reactive gastoward a target made from a material of which the film is to be formed.In this process the target is maintained at reduced pressure in a vacuumchamber and is sputtered by the reactive ion beam. The sputtered targetmaterial is collected as a thin film on a substrate which is physicallyisolated from the plasma generating and the sputtering process.Preferably the reactive gas is present in a mixture of same with aninert gas heavier than the reactive gas the reactive gas being presentin an amount sufficient to passivate at least some of the dangling bondsformed in the sputtered material collected on said substrate.

In contrast to the relatively uncontrolled glow discharge or chemicalvapor deposition and r-f sputtering techniques, ion beam deposition is acomparatively controlled method of thin film preparation providingprecise control of deposition parameters. In both chemical vapordeposition and r-f sputtering the deposition parameters and the actualdeposition are relatively uncontrolled since the film is formed in theimmediate presence of a rather violent energetic plasma. In r.f.sputtering, the sputtering with attendent high substrate temperaturesand the intermixing of film formation and the glow discharge canactually cause damage to the film being formed and as with chemicalvapor deposition results in the incorporation of sputtered gas atoms inthe film. One of the most troublesome difficulties in both of thesetechniques is the control and incorporation of impurities and defects inthe film thereby deleteriously effecting the electrical properties ofthe film. This is manifested by, for example, the presence of danglingbond sites and in the case of amorphous silicon by the coordination ofthe dangling bonds as the mono, di and trihydrides. As describedpreviously the di and trihydrides do not have good photoelectricproperties.

Briefly, the type of thin film preparation according to the presentinvention involves generation of ions from an ion source using electronimpact ionization and extraction and acceleration from this plasma of afocused monoenergetic beam of positive ions using a screen andaccelerator grid electrodes. A monoenergetic beam is one where all ionsstrike the target with about the same energy with a deviation of only afew electron volts. This enables greater control over the sputteringprocess. Control over beam energy from 0 to about 2000 ev with a spreadof 1 to 2 ev is achieved by varying the bias voltage applied to the gridassembly. The grid assembly also serves as a radiation and mass barrierisolating the substrate from the plasma generating process. The vacuumin the deposition chamber is 1 to 2 orders of magnitude higher than inthe ion gun. The accelerated ion beam is directed from the ion gun at atarget which is sputtered. Thin film formation occurs by collectingejected target atoms onto a substrate which sits in a field free regionof space essentially isolated from plasma generation and primary ionbeam processes. With an ion beam flux greater than about 3 milliamps persquare centimeter good film deposition rates are achieved.

In a particular application of the present invention semiconductingfilms of the elements of Group IVa of the Periodic Table and inparticular the hydrogen alloys of silicon, carbon and germanium areprovided. These hydrogen alloys may be of the general formula Si_(x)C_(y) Ge_(z) H.sub.(1-(x+y+z)) where x, y, and z are from 0 to 1. In aspecific application of the ion beam deposition technique the ion beamis collimated providing a large diameter controlled beam which reducesthe potentially extraneous sputtering away from the target. In additionwith the thin film formation physically isolated from both the plasmagenerating process and the sputtering process the opportunity for damageto the growing thin film by these processes is virtually eliminated. Ina particularly preferred application of the present invention anamorphous silicon film is formed on the substrate by sputtering asubstantially pure crystalline silicon target with an ion beamcontaining an inert gas and reactive hydrogen. It is found with such atechnique that the dangling bonds in the amorphous silicon arepassivated and coordinated with hydrogen as the monohydride therebyproviding good photoelectric properties.

Accordingly it is an object of the present invention to provide animproved method of making semiconducting films.

It is an additional object of the present invention to make amorphoussemiconducting films having reduced dangling bond defects.

It is a further object of the present invention to make amorphoussemiconducting films which have improved photoconductive properties.

It is an additional object of the present invention to make amorphoussemiconducting films with few impurities.

It is an additional object of the present invention to form amorphoussemiconducting films on an unheated substrate.

It is a further object of the present invention to providesemiconducting films free of defect states and with reduced microvoidsand inhomogenieties.

It is an additional object of the present present to provide anamorphous silicon film where the dangling bonds are coordinated withhydrogen as the monohydride.

For a better understanding of the invention as well as other objects andfurther features thereof reference is had to the following drawings anddescriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation in cross-section of the apparatusthat may be used to practice the present invention.

FIG. 2 is an enlarged schematic representation in cross-section of theion gun that may be used to practice the present invention.

FIG. 3 is a graphical representation of the change in resistivity withhydrogen concentration in the reactive gas for substrates at ambient andelevated temperatures.

FIG. 4 is a graphical representation of the Fourier transform infraredspectra of a typical film prepared according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to FIG. 1 the method of the present invention will bedescribed in greater detail with reference to the apparatus that can beused to perform it. The stainless steel vacuum chamber 10 is evacuatedthrough conduit 12 by a pump (not shown) to provide a pressure in thechamber of from about 5×10⁻⁹ Torr to about 10⁻³ Torr. The ion beamgenerating device or gun 14 which is a Kaufmann type ion source isattached to the vacuum chamber on a port flange and when the chamber isevacuated the pressure in the ion gun with gas flowing through it isfrom about 10⁻³ to 10⁻² Torr. The Kaufmann type ion source may be moreclearly seen with additional reference to FIG. 2. The ion gun comprisesan anode 16, a screen grid 18 forming an end of the chamber and acathode which comprises a wire filament 22 which is supported by endsupport 24. An array of permanent magnets 26 is placed behind the anodewhich produce a multipole field configuration across the anode andserves to keep electrons emitted by the cathode from traveling directlyto the anode. The magnets serve to have the electrons execute a spiralpattern covering uniform excitement and therefore uniform plasma. Withthis type of field distribution electrons have free access to most ofthe chamber volume producing a very uniform beam. The reactive gas whichmay be a mixture of hydrogen and argon is introduced into the ion gunfrom the gas inlet 28. In front of the cathode is a screen grid 18 andadjacent to it is an accelerating grid 32 each of which has small holesto act to give direction to the ions in the ion beam. In operation thecathode may be controlled independently of the anode with for example acathode emission current of from about 10 to about 15 amperes whichserves to give off electrons which ionize the reactive gas. Thepotential between the anode and the cathode filament is variable and ofthe order of from about 50 to about 2000 volts.

Both the cathode and anode are circular going around the ion gunchamber. During operation a relatively large potential difference existsbetween the anode and the cathode and electrons are boiled off thecathode which generate positive ions from electron bombardment of thereactive gas. The screen grid is biased positively at about the samepotential as the anode and serves to condense or focus positive ionsinto small beams. Since it is biased negatively, the acceleration gridattracts positive ions and since the vacuum in the vacuum chamber isalso attracting these ions, they are accelerated into the vacuumchamber. The net potential that the emitted positive ions are emitted atis the difference between the accelerator grid and the anode potential.

The screen 18 which is made of pyrolytic carbon has small holes of theorder of 0.1 cm in diameter spaced in a uniform pattern. Adjacent thescreen is the accelerator grid 32 which is also made of pyrolytic carbonand also has a uniform pattern of small holes, the holes being offsetslightly relative to the holes in the screen. The screen is at or nearpositive anode potential, (V_(t)) and serves to shape the positive ionsinto small beamlets which are accelerated by the negative potentialV_(a) of up to 500 volts on the accelerator grid. These beamlets emergein the form of a composite ion beam with energy V_(t) -V_(a). Controlover beam energy which is of the order of several thousand electronvolts is achieved by the voltages applied to the anode and acceleratorgrid. The grids also serve as a radiation and mass barrier screening thesubstrate from the plasma in the ion gun and keeping the vacuum in themain chamber at 1 to 2 orders of magnitude higher than that in the ionsource.

The ion beam is in collimated form as it emerges from the grid and isdirected to the target 34 which is supported on plate 36 the back ofwhich is cooled to 20° C. to 30° C. by water running through coils 38.The target is mounted so that it is rotatable thereby providingdifferent incidence angle to the ion beam. Typically however the targetis positioned at the angle of maximum sputtering yield which is fromabout 30° to 45° to the ion beam.

When the ion beam strikes the target, the target is sputtered, and thesputtered target material is sprayed around the vacuum chamber with alarge amount of the sputtered target coming to rest as a thin film onthe substrate, which may be at ambient temperature or which may beelevated up to about 300° C. by the quartz lamps 42. If desired a mask44 may be placed adjacent the substrate so that a pattern of thesputtered material may be defined on the substrate. The substrate may beof any suitable material. If elevated temperatures are desired thesubstrate must be capable of withstanding the temperature. Typically thesubstrate is flexible and is available in large sheets. Typicalmaterials include such diverse materials as aluminum, quartz, stainlesssteel, glass, polyethylene, Teflon, Mylar.

The reactive gas introduced to the ion gun contains at least one gaswhich will be reactive during sputtering with the target material.Typical reactive gases include among others, hydrogen, fluorine,chlorine, nitrogen and oxygen. In a preferred embodiment hydrogen isused particularly where a film of amorphous silicon hydrogen alloy isdesired. Where the reactive gas is relatively low in sputteringefficiency due to poor momentum transfer such as hydrogen, it can bemixed with an inert gas such as argon which serves to bombard the targetwith heavier atoms than the reactive gas thereby increasing thesputtering efficiency and film deposition rate of the process. Hydrogenfor example, is a very light gas which upon striking a target tends tobounce off like a ping pong ball, whereas argon tends to strike thetarget with the force of a ball bearing. Typically the reactive gas ispresent in the gas mixture in an amount up to about 90 percent byvolume.

While the process may be accomplished at elevated temperatures up toabout 300° C., for example, a particular advantage is that it may beperformed at ambient temperature where the substrate is at about 25° C.at the start of deposition and at about 60° C. or less at the conclusionof deposition. This has the advantage of enabling the use of a widevariety of substrates that are sensitive to temperature and therebyproviding a new group of semiconducting materials.

The target may be made of any suitable material. It may, for example, bean amorphous, crystalline or polycrystalline material. Typically theGroup IVa elements such as silicon, germanium, carbon and mixturesthereof are suitable targets. A particularly preferred material is purecrystalline silicon which produces an amorphous silicon film on thesubstrate.

If desired the semiconductor film formed on the substrate may be dopedto provide p and n type semiconductors. The doping may be accomplishedby having a doped target in which the dopant is uniformly dispersed inthe target material or by adding volatile dopants to the sputtering gas.Alternatively a split target of target material and dopant material maybe used. Typical p type dopants include boron and typical n type dopantsinclude, phosphorous, antimony and arsenic.

For optimum results in minimizing the presence or occurrence ofimpurities in the semiconducting film produced as a result of thesputtering process it is preferred to place a shield in the vacuumchamber between the stray ion beam and the vacuum chamber surface. Evenwith a collimated ion beam some small portion of the beam may not strikethe target but rather may overshoot the target and sputter the stainlesssteel vacuum chamber. Further some of the ions in the ion beam strikingthe target may be deflected by the target and sputter the chamber walls.Furthermore, any other implements that may be present in the chamber maybe sputtered if the ion beam strikes it. These sputterings areundesirable since they may provide a presence of impurities from thesputtered chamber or implements in the film being formed. To minimizethis deleterious sputtering of the chamber a shield is placed behind andaround the target in the path of stray ion beams before they strike thechamber walls. Typically this shield 46 protects the entire interior ofthe vacuum chamber as shown in FIG. 1. In addition, if any implementsare present in the chamber they may be coated or shielded for the samepurpose. The shield is typically made of a material that will notintroduce electrically active band gap states in the film beingdeposited and that will have a low sputtering efficiency compared to thetarget. Carbon is an ideal shield material for silicon since it is aGroup IV element and does not introduce electrically active states intothe band gap of silicon. Typically the sputtering efficiency is an orderof magnitude lower than the target material. For a 500 electron voltargon beam at 1 milliamp per square centimeter, silicon, germanium andcarbon have sputtering efficiencies of 400, 900 and 40 angstroms perminute respectively. Under the same conditions the sputtering efficiencyof stainless steel of which the chamber is made is 250 angstroms perminute. The shield material preferably has good operating properties ina vacuum, will withstand high temperature, will not vaporize and has lowoutgassing or low vapor pressure properties. A particularly satisfactoryshield material is a thermally shocked graphitic form of carbon called"Stackfoil" which is marketed by the Stackpole Carbon Co., in SaintMary' s, Pa. This material which is an expanded form of graphiteproduced by heating graphite very quickly to temperatures over 1000° C.is 99.9% pure and is available in large flexible sheets which can bereadily shaped to conform to a surface to be shielded from the stray ionbeam.

EXAMPLES

A target which comprises a six inch slice of high purity, (greater than99.9999% pure), undoped polycrystalline silicon is heat bonded to acopper backing plate. The ion gun is mounted on part of the high vacuum(5×10⁻⁹ Torr) stainless steel chamber which is pumped by a liquidnitrogen trapped diffusion pump. Prior to deposition the system ispumped down to 5×10⁻⁷ Torr and during deposition a vacuum of 10⁻⁴ Torris maintained. The target is sputtered with a reactive ion beam of argonand hydrogen of a high flux (3 milliamps per square centimeter orgreater) produced from the ion gun. The hydrogen content in the reactivegas mixture of hydrogen and argon is varied up to about 90 percent byvolume and test runs are made as indicated in FIG. 3. The ion beamenergy is 500 electron volts, the acceleration voltage is 200 V, thedischarge voltage is 50 and the cathode current is about 12 amperes.Examples are prepared as a function of hydrogen concentration in the ionbeam and substrate temperature and the results are graphicallyillustrated in FIG. 3. Deposition rates depend on the concentration ofhydrogen in the ion beam and vary from about 3 microns per hour withessentially pure argon to about 0.6 microns per hour with 75 percent byvolume hydrogen in the ion beam.

FIG. 3 illustrates the results of dark resistivity measurements. To makethese measurements films are deposited on quartz substrates whichcontain a photolithographically printed pattern of 150 angstroms thickchromium electrodes arranged for four probe resistivity measurements.The geometry of the electrodes is chosen so that very linear currentsand field profiles are obtained in a thin film. As illustrated in FIG. 3the resistivity at any given deposition temperature varies dramaticallywith hydrogen concentration increasing over 6 to 7 orders of magnitudefrom 0 to 75 percent by volume hydrogen in the reactive gas mixture. Itcan also be seen that increasing the substrate deposition temperaturefrom room temperature to about 200° C. has a smaller effect onresistivity and that the single test at a substrate temperature of 300°C. shows very little improvement over the resistivity at 200° C.

From SIMS (Secondary Ion Mass Spectroscopy) which measures hydrogen andimpurity concentration and Fourier transform infrared spectroscopyhydrogen incorporation and bonding to silicon in the thin films whichare produced is evident. For examples prepared with hydrogen ion beamSIMS data shows a strong peak at m/e=1 indicative of hydrogen whereasthis feature is absent in unhydrogenated films. The Fourier transforminfrared spectra typical of an example is shown in FIG. 4. Excitation ofvibrations characteristics of silicon-hydrogen bonding are evident inthe 2000 cm⁻¹ and 630 cm⁻¹ peaks which are usually assigned tostretching and bending modes of Si-H. The peak at 2000 cm⁻¹ correspondsto the silicon hydrogen bond existing as the monohydride. The absence ofany discrete peak at 2100 cm⁻¹ indicates that there is little, if any,multihydride or polysilane type of silicon-hydrogen coordination presentin the films. This is important since as previously discussed thepresence of multihydride is believed to deleteriously effect theelectrical properties of the film.

From both X-rays and selected area electron diffraction characterizationthe structure of the films is determined to be amorphous. Chemicalcomposition is checked using X-ray fluorescence spectroscopy and SIMSand no evidence of metallic impurities could be detected down to thedetection limit of the analysis, 10 ppm. A hydrogenated film is alsoexamined using phase contrast transmission electron microscopy. No voidnetwork or any other structure is seen down to the resolution of themicroscopy. This indicates that there is no unsatisfied bonds oninternal film surfaces giving rise to localized band gap defects.

In summary ion beam deposition differs from r.f. sputtering and glowdischarge deposition in that with ion beam deposition it is possible tomore precisely control the deposition process. This comes about becauseof the use of an ion gun to generate the sputtering species and becauseion generation, sputtering and thin film formation are completelyindependent. The employment of an ion gun permits independent controlover the energy and current density of the bombarding ions. Collimationof the beam by the grids isolates the substrate from the ion beamminimizing the probability of process induced defects. With ion beamsources, deposition is done at very low working gas pressures of theorder of 10⁻⁴ to 10⁻⁵ Torr producing high purity thin films. Because iongeneration sputtering and film formation are completely decoupled andthe angle of incidence of the target to the ion beam and of thesubstrate to the sputtered species is completely variable maximumcontrol over sputtering efficiency and thin film growth may be achieved.

In conclusion, a new method for preparing semiconducting films has beenprovided. In particular a new method of making amorphous semiconductingfilms of the Group IV elements by a reactive ion beam deposition isprovided. Ion beam deposition with reactive gas such as hydrogendramatically minimizes or eliminates the dangling bonds in the filmswhich heretofore gave rise to defects in the band gap and sites forcoordination with impurities. Ion beam deposition where the substrate isphysically isolated from the plasma generating and primary sputteringprocess markedly reduces the possibility of impurites being present inthe film. In a particular preferred embodiment amorphous siliconmonohydride film with excellent photoelectrical properties is prepared.

While this invention has been described with reference to the specificembodiments disclosed it will be apparent to those skilled in the artthat many alternatives, modification or variations may be made by thoseskilled in the art. It is intended to embrace all such alternatives andmodifications as may fall within the spirit and scope of the appendedclaims.

We claim:
 1. A method for producing semiconducting films on a substratecomprising generating a plasma of reactive gas, extracting, acceleratingand directing an ion beam from said plasma toward a target of materialof which the film is to be formed, said target being maintained in avacuum chamber at reduced pressure, sputtering said target with saidreactive ion beam to sputter the target material, collecting saidsputtered target material as a film on said substrate, said substratebeing physically isolated from the plasma generating process and thesputtering process.
 2. The method of claim 1 wherein said reactive gasis present in a mixture of said reactive gas with an inert gas heavierthan said reactive gas.
 3. The method of claim 1 wherein said reactivegas is hydrogen.
 4. The method of claim 1 wherein said semiconductingfilm and said target material comprise the same element or elements fromGroup IVa of the Periodic Table.
 5. The method of claim 4 wherein saidsemiconducting film is amorphous silicon and said target materialcomprises substantially pure crystalline silicon.
 6. The method of claim5 wherein said reactive gas is hydrogen and said amorphous film ofsilicon has silicon atoms coordinated with hydrogen substantiallycompletely as the monohydride.
 7. The method of claim 1 wherein said ionbeam is collimated.
 8. The method of claim 1 wherein said target is anamorphous polycrystalline or crystalline material.
 9. The method ofclaim 1 wherein the substrate is at ambient temperature.
 10. The methodof claim 1 wherein the substrate is maintained at temperature of fromabout 25° C. to about 300° C.
 11. The method of claim 2 wherein saidinert gas is argon.
 12. The method of claim 3 wherein said reactivehydrogen is present in said plasma in an amount of up to about 90percent by volume the remainder being an inert gas heavier than thehydrogen.
 13. The method of claim 1 wherein said substrate is flexible.14. The method of claim 2 wherein said reactive gas is present in anamount sufficient to passivate some of the dangling bonds formed in thesputtered material collected on said substrate.
 15. The method of claim4 wherein said amorphous silicon is deposited at a rate of from about0.6 microns per hour to about 3.0 microns per hour.
 16. The method ofclaim 4 wherein the pressure in the vacuum chamber during sputtering isfrom about 5×10⁻⁹ Torr to about 10⁻³ Torr.